The present invention relates to a mobile communication system radio channel selection method and system for assigning radio channels for communication to mobile terminals having a plurality of different communication schemes so as to perform line connection with respect to the mobile terminals.
With recent advances in multi-vender systems and open network systems in the field of mobile communication, systems in which apparatuses for performing switching control and radio channel management are respectively shared in mobile switching centers and radio base stations have been constructed. In addition, half-rate speech codec techniques of decreasing the coding bit rate to accommodate more subscribers and realize effective use of radio frequencies have been developed.
A system using two communication schemes (radio channel access schemes), i.e., a TDMA (Time Division Multiple Access) scheme designed for 3-channel multiplexing per carrier (to be referred to as a full rate scheme hereinafter) and a TDMA scheme designed for 6-channel multiplexing per carrier (to be referred to as a half rate scheme hereinafter) is expected to be introduced in the near future. The full rate scheme uses a full rate codec scheme with a coding bit rate of 11.2 kbps. The half rate scheme uses a half rate codec scheme with a coding bit rate of 5.6 kbps. The two schemes, however, are not compatible with each other because of different speech coding algorithms and different error correction schemes. Therefore, radio channel management which can cope with both the communication schemes is required, in addition to conventional radio channel management.
FIG. 5 shows the system disclosed in Japanese Patent Laid-Open No. 4-323921, which is used to explain a conventional channel selection technique. Mobile terminals 3a to 3d are respectively connected to lower line connection units 2a and 2b, which are connected to a host line connection unit 1. When a shortage of frequencies managed by the lower line connection units 2a and 2b occurs, frequencies managed by the upper line connection unit 1 are assigned to lower line connection units 2a and 2b in which the frequency shortage has occurred.
If the mobile terminals 3a to 3d are designed to selectively use the full rate scheme and the half rate scheme, frequency management can be easily performed by applying the method shown in FIG. 5.
Frequency management, however, must be performed not only in the upper line connection unit 1 but also in the lower line connection units 2a and 2b, resulting in a complicated arrangement. Assume that frequency management is to be performed only in the lower line connection units 2a and 2b to prevent the above problem. In this case, since the full rate scheme and the half rate scheme are not compatible with each other, as described above, frequencies must be assigned and fixed to the full rate scheme and the half rate scheme, respectively. In this case, if no available channel is present in one scheme, any available channel in the other scheme cannot be used.